Hybrid metal oxide—particularly silica—particles involve covalent bonds between organic and inorganic components within the particles. Hybrid silica particles in particular have become very popular high pressure liquid chromatography (“HPLC”) packing materials for HPLC columns because of the particles' stability at high pH and high physical strength.
One general method for preparing hybrid totally porous silica particles is an emulsion polymerization method (see, e.g., U.S. Pat. Nos. 4,017,528, 6,686,035, and 7,223,473 and WO2006039507). This method involves emulsion polymerization of an organic siloxane polymer precursor in emulsion droplets in the presence of polyethylene glycol (“PEG”) or toluene as a porogen, in which the siloxane polymer precursor contains an organic moiety such as a methyl group or ethylene bridging group. The PEG or toluene is later washed out, reducing or eliminating the need for high temperature burn-off.
In 1999, porous hybrid periodic meso-organosilicas (“PMOs”) were introduced by various research groups using a micelle-templating method (e.g., Inagaki, et al., J. Am. Chem. Soc., 121, 9611, 1999; Melde, et al., Chem. Mater., 11, 3302, 1999; Ishii, et al., Chem. Commun., 2539, 1999). The mechanism of forming the pore structure is based on self assembly of surfactants described in U.S. Pat. No. 5,098,684 where highly ordered mesoporous silica with high surface area was reported. The relatively uniformly sized pores formed after removing the surfactants were generally in an ordered periodic arrangement. PMOs have the advantage of micelle-templated mesoporous materials such as ordered pore structures and high specific surface areas but also that of hybrid materials such as high pH stability and high physical strength. However, the porous materials mentioned above all have irregular shapes which limit their applications in chromatography.
Very recently, some have attempted to synthesize spherical PMO particles for chromatography applications. However, to control the PMO particle morphology, particle size, particle size distribution, and pore size and pore size distribution simultaneously and precisely is still a great challenge. Kapoor and Inagaki reportedly prepared phenylene-bridged PMO particles with a pore size of 2 nm and particle size distributed from 0.6 to 1.0 μm (Kapoor, M. P., Inagaki, S., Chemistry Letters, 33, 88, 2004). Rebbin et al. reportedly prepared ethane-bridged PMO particles with an average size of 0.4˜0.5 μm and a pore size of 3 nm (Rebbin et al., Micro. Meso. Mater. 72, 99, 2004). Kim reportedly synthesized ethane-bridged PMO particles with sizes from 1.5 to 2.5 μm and an average pore sizes of 3.2 nm by microwave heating. However, PMO particle size distributions are poor according to SEM images (see, Kim et al., Chemistry Letters, 33, 422, 2004).
Ultimately, monodisperse particles having average sizes between 1.0 and 5.0 μm and pore sizes larger than 5.0 nm are needed for HPLC applications. In addition, narrow particle size and pore size distributions are preferred. So far, there are no methods of making totally porous PMO particles that can meet these requirements.
Another method known as pseudomorphic transformation was proposed by
Martin to make ordered mesoporous pure silica spheres (see Angew. Chem. Int. Ed., 41 (2002) 2590). Pseudomorphism is a term used by mineralogists to describe phase transformation that does not change the shape of a material. Pseudomorphic synthesis, assisted by surfactant, for mesoporous pre-shaped silica particles can form a highly ordered narrow mesopore size distribution, high specific surface areas and pore volumes without changing the initial shapes of the silica particles. For example, K. Unger reportedly synthesized 10 μm totally porous pure silica particles with pore diameter ranging from 7 to 9 nm, specific surface area of 900 m2/g, and pore volume of 1.5 ml/g (“Synthesis of Large-Pore Mesostructured Micelle-Templated Silicas as Discrete Spheres”, Chem. Mater., 2005, 17, 601-607).